For representing the transmission ratios, automatic transmissions for motor vehicles comprise several shift elements, which can, for example, transfer torque with hydraulic pressure. With an amount of the pressure, which is also known as the clutch pressure, the halves of the shift elements are pressed against each other with an increasing force if the shift element is disengaged in a pressureless state. Due to this, the torque that can be transferred by the frictionally engaging shift element increases, or the shift element transitions from a disengaged state into the state of torque transfer subject to slip, that is, with dynamic friction. The capability to transfer torque is designated in the following as the transfer capability. An increasing transfer capability is therefore understood to be an increase of the torque that can be transferred by the shift element. If the pressure acting on the shift element increases further, the dynamic frictional torque increases and with it the transferable torque.
This applies analogously to shift elements which are actuated by a different acting energy, for instance, frictionally engaging shift elements that are engaged electromechanically.
This is also the case with frictionally engaging shift elements that are engaged without pressure, for instance using the effects of spring force. When the frictionally engaging shift element is disengaged due to pressure, this pressure is reduced for engaging the shift element, and then starting from a specific pressure, the spring force moves the halves of the shift element against each other. The transfer capability starts analogously to the case of a shift element that is disengaged when not under pressure.
Automatic transmissions typically comprise a hydrodynamic startup element, for instance a hydrodynamic torque converter that is disposed between the drive motor and a transmission. The torque converter, in addition to a smooth startup procedure, advantageously creates an increased torque during startup of the motor vehicle. A hydrodynamic clutch that exclusively guarantees a smooth startup procedure is used less often in a motor vehicle. Both hydrodynamic startup elements comprise a drive-side pump impeller that is connected in a rotationally fixed manner to the drive motor, and an output-side turbine rotor that is a coupled in a rotationally fixed manner to a transmission input shaft, for example. The driven pump impeller transfers momentum to an operating medium, generally oil, which is further transferred to the output-side turbine rotor. Common to both startup elements is the slip, or the speed difference between the pump impeller and the turbine rotor, which cannot be coupled together in a rotationally fixed manner, because torque transfer occurs exclusively by means of a hydrodynamic momentum exchange. During the startup procedure, in which a drive motor rotating at a specific rotational speed, is to be coupled via a stationary transmission, or drive train, to the wheels of the vehicle, without torque impulses to the greatest extent possible, the slip is necessary to perform the coupling function. After the startup procedure, however, slip is undesirable because this represents an undesired loss of power, and disadvantageously lowers the efficiency of the drive. For this reason, a clutch element, which is also designated as a converter lock-up clutch, is disposed between the pump impeller and the turbine wheel. By engaging the converter lock-up clutch, it is possible to connect the pump impeller and the turbine rotor together in a rotationally fixed manner, and thus to increase the efficiency of the drive train.
To guarantee a steady increase of the transfer capability of a frictionally engaging shift element, the respective pressures are increased in a defined ramp-like progression. The pressure is set using an electronic transmission control unit (EGS) that issues an electrical current as a control variable, and thereby controls an electrohydraulic gearshift device (HSG), which thereupon sets a specific pressure, depending on the value of the current, by means of an electrical pressure regulator (EDS). It should be noted that the current-pressure correlation can be different for each pressure regulator due to the manufacturing tolerances of the pressure regulator.
In order to be able to reproducibly set the desired pressure ramp it is necessary to have precise knowledge of the correlation of current which controls the EDS, and pressure generated by the EDS.
The value of the current required for generating a specific clutch pressure can be determined from a table stored in the EGS, or calculated from the desired clutch pressure using a mathematical function. The mathematical function is a polynomial, for example. Specific characteristic values, known also as compensation data, must be known in order to define the polynomial, or the table; for example, a fill current is issued by the EGS, and supplied to the electrical pressure regulator in the hydraulic gearshift device, which then generates a fill pressure in the hydraulic system or in the shift elements. The fill pressure is attained specifically when the shift element, for example a hydraulic clutch or brake, is filled to the extent that a specific pressure has built up, and the friction surfaces of the shift element have approached each other to the extent that there is dynamic friction between them, and thus torque begins to transfer.
The fill pressure, which can differ for each shift element within an automatic transmission depending on the geometric shape, is therefore assigned a specific value of the fill current. In this context, the experimentally determined fill pressure is identical for all shift elements of the same shape, even if these elements are disposed in different transmissions of the same type.
Transmission concepts are known that have a mechatronic. A mechatronic is a permanently installed assembly that substantially comprises an HSG and an EGS. It is possible to compensate for tolerances in the current-pressure correlation of the electrical pressure regulators, for example, by means of compensation data handling, and thereby to be able to use inexpensive pressure regulators for high shift quality of new vehicles.
In contrast, in the case of separate HSG and EGS components, such compensation data handling is not possible in the event of a subsequent exchange of one or both components in the field.
A method for determining characteristic values of an automatic transmission is known from the document DE19643305 A1. For this, an automatic transmission on a final test bench is shifted into the individual transmission ratio steps, where an input and output transmission speed, as well as an input and output transmission torque are measured. Characteristic values of the automatic transmission such as fill time, fill pressure, reaction time or friction value of the disks are determined from these measured values for the clutch to be engaged during shifting. These characteristic values are then stored in a memory so that an electronic transmission control device corrects the pressure level and the time of a rapid filling pressure, fill pressure and the pressure level of a shift pressure based on these characteristic values. Using this method, the fill pressure, for example, is determined in that, with the automatic transmission powered, the pressure level in the respective shift element is increased until the output torque exceeds a defined limit value.
A disadvantage here is that torque measurement requires expensive measurement equipment and sensors. In addition, such measurement can only occur on a test bench using a transmission that has been removed from the vehicle, which is not service-friendly, and is expensive with respect to the installation effort and costs for an application in the field or service sector.